Hydraulic control systems

ABSTRACT

A hydraulic control system for a continuously-variable-ratio-vehicle transmission of the toroidal-race rolling-traction type, in which the mechanism for positioning each roller includes a double-acting ram (11). The two opposed faces of the ram are exposed in normal use to the pressures of two separate lines of fluid (21, 23), each being pressure-controlled by separate valves (32, 34) located downstream of the respective ram face. The outputs of those separate valves combine in a common region (31), and further valve means (40, 41) open to connect that region to each line in response to the occurrence of a predetermined difference in pressure between the region and that line. In such circumstances such connection and the resulting transfer of fluid from the region to the line helps to counteract any tendency to fluid starvation in the line. This might otherwise occur for instance in response to the rapid movement of all the double-acting rams in the same direction, for example when the vehicle is approaching wheel-lock during emergency braking.

This invention relates to hydraulic control systems forcontinuously-variable-ratio transmissions ("CVT's") of the toroidal-racerolling-traction type. It relates in particular to such transmissions inwhich so-called "hydraulic roller control" is used, that is to saytransmissions in which the rollers are mounted on or otherwise connectedto double-acting hydraulic rams, the opposite ends or faces of which areexposed to separate flows of fluid each at controlled pressure. Movementof the ram back and forth causes the roller centre to move back andforth along the centre circle of the common torus presented by the inputand output races, and so to change the transmitted ratio in a mannerwell understood in the art.

BACKGROUND OF THE INVENTION

Such a hydraulic control mechanism is shown and described, instance, inpatent specification EP-B-0133330. More particularly still, theinvention applies to a control system for a transmission having aplurality of rollers, typically six, each mounted on an individual ramand all of which must transmit the same ratio at any one time. One ofthose rollers, known in the art as the "master roller", is mounted on adouble-acting ram the two cylinders of which are in direct connectionwith the control valves by which the two fluid pressures are controlled,while the fluid in the corresponding ram cylinders of each of the otherrollers (known as the "slave rollers") is simply in communication withthe cylinders of the master roller and so at a common pressure. Acontrol circuit for such a combination of the master and slave rollersis also described in specification EP-B-0133330.

When a transmission with such a control system is working for the timebeing at normal rates of ratio change, the roller control rams will allbe moving slowly within their respective cylinders. The separate flowsof fluid down the two sides of the system-that is to say a first sideincluding all the cylinders at one end of the respective rams, and asecond side including all the cylinders at the other end-need only besufficient to maintain the cylinders full on that side of the systemwhere the cylinder volumes are increasing. A flow rate of a maximum ofsay 10-15 L/min is typical for vehicle transmissions, so that thehydraulic power source of the system need only comprise two small pumpseach capable of delivering that flow, or alternatively a single pumpcapable of delivering say 20-30 L/min and fitted with a flow divider.However, when such a transmission undergoes a very rapid change ofratio, for instance when fitted to a vehicle which is approachingwheel-lock during emergency braking, all the roller control rams moverapidly in the same direction. The volume avalilable for fluid in thecylinders on one side of the system therefore increases rapidly,requiring a power source capable of delivering a considerable flow inorder to maintain a reasonable minimum pressure within the cylinders or,in an extreme case, to avoid cavitation within them. To meet suchconditions, it has therefore been customary to provide such hydrauliccontrol systems with power sources far larger and more powerful than isnecessary while the rams are either stationary or moving slowly, as isthe case during most of the working life of the transmission.

SUMMARY OF THE INVENTION

The present invention arises rom appreciating that such conditions tendto create a substantial fluid pressure difference between the region ofthe circuit that lies downstream of the control valves, and the upstreamside of the system where cylinder volumes are increasing, and that thispressure difference may be used to trigger the transfer of fluid to theupstream, low pressure, side. The tendency to fluid starvation on thelow pressure side is therefore diminished, and with it the maximum flowrate demanded of the source and so the size and expense of the sourceitself. Means for effecting such transfer are quite absent in priorpublications such as patent specification EP-B-0133330 and the relatedEP-B-0078125 where the it outlets of the respective controlvalves--identified by reference 23 in both specifications--lead only totank or exhaust, and there is no means of connection between thoseoutlets and upstream regions of the hydraulic circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is defined by the claims, the contents of which are to beread as included within the disclosure of this specification, andincludes hydraulic control systems as described with reference to theaccompanying drawings. The invention will now be described by way ofexample with reference to those drawings in which:

FIG. 1 is a schematic view of a hydraulic control circuit for a vehicleCVT, and

FIG. 2 is a partial view in the direction of the arrow II in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1 and 2, roller 1 is one rollers that transmit traction betweenan input and an output disc of a transmission of the toroidal-racerolling-traction type. FIG. 2 shows input and output discs 2 and 3rotatable about a common axis 4, with roller I rotatable about an axle 5and in rolling contact with part-toroidal races 6 and 7 of discs 2 and 3respectively. As is well known in the art, by tilting roller 1 in thedirection indicated by arrow 8 the transmitted ratio is changed.

As FIG. 1 shows, the axle 5 of roller 1 is mounted in bearings 10 in ashaft 11 carrying pistons 12, 13 at its opposite ends. Shaft 11 is alsoformed with a slot 14, providing clearance within which the rollerrotates. Shaft 11 and pistons 12, 13 form parts of a double-ended ramunit which is indicated generally by reference 15. Pistons 12, 13 movewithin cylinders 16, 17 respectively. Cylinders 16 and 17 are connectedto opposite sides 18, 19 of a hydraulic circuit. Side 18 receives thedelivery of a pump 20 and is connected to the inlet 21 of cylinder 16,and side 19 receives the delivery of a pump 22 and is connected to theinlet 23 of cylinder 17. Pistons 12, 13 can rotate about their commonaxis 24 within their cylinders 16 and 17. As is well known in the art,under such control systems the roller I seeks a ratio angle--that is tosay a particular angular setting within the range indicated by arrow8--at which the reaction experienced at its contacts with races 6, 7 isbalanced, in the direction of axis 24, by the net force exerted incylinders 16 and 17. A change in the reactions at the races, and/or achange in the net force exerted from the two cylinders, results both inmovement of the ram 15 along axis 24 and rotation about it, until roller1 reaches a new angular setting at which equilibrium is restored.

Outlet 30 of cylinder 16 leads to a gallery 31 by way of a proportionalpressure control valve 32, and outlet 33 of cylinder 17 leads to gallery31 by way of a similar proportional control valve 34. The valves 32 and34 are controlled by the electrical output of a micro-processor 35 whichresponds typically to many inputs, including for instance engine speed,ratio angle, oil temperature etc. but especially to demand by the driverof the vehicle arid which causes the valves 32, 34 to regulate the fluidpressures in cylinders 16, 17 so as to achieve the appropriate ratiosetting of roller 1. Fluid passes from gallery 31 both to a lubricationcircuit 36, and to a drain 37 by way of a pressure relief valve 38.

According to the invention the common downstream gallery 31 is connectedto side 18 of the circuit by a non-return valve 40, and to side 19 by anon-return valve 41. In normal use of the transmission, with ram 15either stationary or in only slow movement, the use of two moderatesized units as pumps 20, 22 will be sufficient to maintain the two sides18, 19 and cylinders 16, 17 full of fluid, and to enable valves 32, 34to work under the control of micro-processor 35 to achieve the desiredpressure levels. However if ram 15 moves quickly, due for example toemergency braking of the vehicle and a consequent rise in the reactionsbetween roller I and races 6 and 7, pressure changes follow. Assume thatram 15 moves to the right as shown in FIG. 1, so that the fluid capacityof cylinder 16 rises suddenly. A pressure fall on the left-hand side 18of the circuit, or in an extreme case cavitation within cylinder 16, isthreatened unless the capacity of pump 20 is much greater than normalconditions would require. However, the same sudden movement of ram 15diminishes the fluid capacity of cylinder 17, and so generates apressure rise on the right-hand side 19 of the circuit. Fluid istherefore discharged through outlet 33 and valve 34 into gallery 31.Non-return valve 40 is set to open when the pressure in gallery 31achieves a desired relationship--for instance equality, or excess by apredetermined amount--with the upstream pressure in side 18. This allowsfluid from gallery 31 to enter that side and so provides the delivery ofpump 20 with the augmentation it requires to meet the abnormalconditions that have occurred. Normal conditions are restored when thesaid relationship between the pressures in gallery 31 and side 18ceases, so that non-return valve 40 closes again.

If the sudden movement of ram 15 had been from right to left, valve 40would of course have remained shut and non-return valve 41 would haveopened, allowing fluid from gallery 31 to enter side 19 and augment thedelivery of pump 22.

It should be noted that the fluid which passes through valves 40 and 41in such abnormal condition must come from gallery 31, downstream of theproportional control valves 32 and 34, and not directly from the outlets(30 or 33) of cylinders 16 and 17; any direct connection between thoseoutlets and the non-return valves would simply prevent micro-processor35 from controlling valves 32 and 34 in normal conditions as well asabnormal.

Typically ram 15 is not the only ram in the system, but is instead the"master" ram to which one or more "slaves" are related. As has alreadybeen mentioned, a transmission of the kind shown in specificationEP-B-0133330 has six rollers and therefore six rams. FIG. 1 shows apossible arrangement of those six rams, with ram 15 the "master" andrams 45-49 the "slaves". The opposed cylinders of rams 45-49 have inlets50-54 and 55-59 respectively: inlets 50-54 connect to side 18 of thecircuit and inlets 55-59 to side 19. Master ram 15 and slaves 45, 46form one group controlling the three rollers which transmit tractionbetween one pair of input and output races, and the other three slaverams 47-49 control the three further rollers which transmit tractionbetween the second pair of races. However, because the cylinders of allthe slave rams 45-49 49 are in supply-side communication with thecorresponding cylinders of the master ram 15, a common pressure existsin them all, and in the event of abnormal conditions the non-returnvalves 40, 41 open to admit an augmenting flow both to one of the masterram cylinders and to all the corresponding slave cylinders associatedwith it.

It might in practice be difficult to provide a single non-return valve(40, 41) on each side of the circuit capable of accommodating the suddenand considerable recirculatory flow of fluid that abnormal conditionsmay require. Greater flows could be made possible, for instance, invarious ways. For instance by substituting for the single non-returnvalve 40 a pair of valves 60, 61 mounted in conduits 62, 63respectively, as shown in broken lines in FIG. 1. Conduit 62 nowconnects gallery 31 to the inlets of rams 15, 45 and 46, while conduit63 provides a separate, second connection between the gallery and rams47-49. On the other side of the system, valves 64, 65 in conduits 66, 67would similarly be substituted for the single valve 41. To permit evengreater flows, it would of course be possible to connect each individualram inlet 21, 50-54, 23 and 55-59 to gallery 31 by way of a separatenon-return valve, so using twelve such non-return valves in all.

It should be particularly noted that the net flow of fluid through thesystem to lubrication 36/drain 37 is constant at all times, being equalto the aggregate 2Q of the output of two equal pumps 20, 22 eachproducing a flow Q. In abnormal conditions, when there is arecirculatory flow of say q from one side of the circuit to the other,the net through flow produced by one side of the circuit will be Q+q,and the corresponding quantity on the other side will be Q-q, so thatthe aggregate delivered to lubrication 36 and drain 37 will still be 2Q.

I claim:
 1. A hydraulic control system for a continuously-variable-ratio transmission of the toroidal-race rolling-traction type, said hydraulic control system comprising at least a first double-acting ram supporting a first roller, said first double-acting ram having two opposed faces which are exposed, in normal use, to pressurized fluid being supplied via at least one pump and two separate fluid supply lines, each said fluid supply line being pressure-controlled by a separate pressure control valve located downstream of the respective ram face, and the discharge from both of said pressure control valves being combined in a common region,wherein return means are provided for returning pressurized fluid from said common region to a desired one of said fluid supply lines and, when a present difference in fluid pressure between a desired one of said fluid supply lines and said common region occurs, said return means opens and allows pressurized fluid flow from said common region to a desired one of said fluid supply lines.
 2. A hydraulic control system according to claim 1, wherein said return means comprises at least one non-return valve for each said fluid supply line, and said at least one non-return valve is set to open at a predetermined pressure level.
 3. A hydraulic control system according to claim 1, wherein said return means comprises at least three non-return valves for each said fluid supply line, each said at least three non-return valves are set to open at a predetermined pressure level.
 4. A hydraulic control system according to claim 1, wherein at least one further double-acting ram, supporting a further roller, is arranged in parallel with said first double-acting ram such that fluid pressure exerted against the corresponding opposed faces of said parallel double-acting rams are substantially similar at all times.
 5. A hydraulic control system according to claim 1, wherein five further double-acting rams, each supporting a further roller, are arranged in parallel with said first double-acting ram such that fluid pressure exerted against the corresponding opposed faces of said parallel double-acting rams are substantially similar at all times.
 6. A hydraulic control system according to claim 1, wherein each said fluid supply line has a separate pump and a micro-processor is connected to and controls and pressure control valves. 